The invention concerns a workpiece support system having a driven, circulating traction means; a stationary base frame; and at least one workpiece support that comprises a load carrier and a traction carriage.
Workpiece support systems are used for transporting workpieces to various work stations, for example, in case of a production line. Workpiece support systems are known that in accumulation operation are driven by means of several advancing devices with loosely driven workpiece supports. In this connection, the topside of the workpiece supports can always stay face up. These workpiece supports either move adjacent to one another (the workpiece supports are moved in one plane), above one another (the workpiece supports move in several planes), or in special cases are comprised of a combination system (the workpiece supports move, for example, in one plane and are lifted by a lifting system into further planes). For this purpose, for each plane (exception: gravity roll table) and for each lifting system one drive is required, respectively.
DE 1718135 U discloses a workpiece support system in which the workpiece support is configured as a carriage running on rails and connected to an endless transport belt. Once the carriages arrive at the end of the production line, they are returned to the start of the production line. The return is realized by means of a bridge extending across the length of the production line. The bridge is arranged at a slant from the end of the line to the start of the line and is supported by two stands that are arranged at the end and the start of the line. The stands comprise lifts as a lifting system whose task it is to lift the carriages at the end of the production line to the return bridge and to lower them onto the production line after crossing the bridge. The workpiece supports thus move in different planes so that a lifting system must be used. The lifting systems each require a drive so that this configuration of a workpiece support system is expensive and complex.
A further workpiece support system disclosed in DE 30 29 620 A comprises a machine frame in which an endless, driven traction means, in particular a chain, is guided by means of which the workpiece supports supported thereon are entrained primarily by friction. The chain is deflected by two deflection locations wherein one of the deflection locations is driven.
In workpiece support systems it is generally a problem that accumulation of workpiece supports can occur. In such a situation the workpiece supports should be decoupled from the traction means. DE 94 21 998 U1 discloses a workpiece support system in which the workpiece support can be disengaged from the traction means. For this purpose, stationary holding and release elements are provided by means of which the workpiece supports that are passing by and are moved by friction by means of the traction means are disengaged from the traction means. By means of the holding and release elements the workpiece supports are vertically lifted so that friction between workpiece support and traction means is canceled.
CH 669 167 A5 discloses a workpiece support system operating only in one plane in which a traction means engages by a friction shoe a workpiece support. In order to prevent that the entire weight of the workpiece and the workpiece support will load the traction means, it is provided that an adjustable load portion is transmitted via wheels onto a support path. The chain as a traction means is moving in a U-shaped guide. The friction shoe of the workpiece support is pressed by a spring load of a predetermined force against the traction means while the residual load is transmitted by the wheels of the workpiece support onto the support path. Moreover, means for reducing or canceling friction are provided that operate automatically when a workpiece support bumps against another one, for example, in case of accumulation.
The known systems do not describe a workpiece support system with forward travel stretch and return stretch in two planes that, by means of only one drive, provides safe deflection of the workpiece supports together with the traction means at the deflection points as well as a defined friction in normal operation between traction means and workpiece support while, at the same time, providing safe operation in accumulation operation. It is the object of the invention to design such a workpiece support system.